AA

Chapter 6: Sleep

In this Chapter:

  • Brain Activity during Sleep
  • Sleep disorders
  • How is sleep regulated?
  • The Sleep-Wakefulness Cycle

Introduction

  • Lack of sleep increases the risk of many health problems including:
    • Diabetes
    • Cardiovascular disease
    • Heart attacks
    • Stroke
    • Depression
    • High Blood Pressure/Hypertension
    • Obesity
    • Infections
  • Sleep is made of several different stages that are accompanied by daily rhythms in hormones, body temperature, etc.
  • Sleep disorders are one of the least recognized sources of disease, disability, and death

Brain Activity During Sleep

  • Electroencephalography (EEG): the measurement of electrical activity in different parts of the brain and the recording of such activity as a visual trace (on paper or on an oscilloscope screen).
  • One sleep cycle is approximately 90 minutes
  • Each night, the brain progresses through a series of stages when brain waves slow down for 60 minutes in NREM (non-rapid eye movement) sleep
    • This is accompanied by the relaxation of muscles and eyes
    • Heart rate, Blood pressure, and body temperature fall
    • If awakened in this stage of sleep, most people only recall fragments of thought
    • NREM sleep: sleep where the eyes do not rapidly move
  • Over the next half-hour, brain activity changes to REM sleep
    • REM (Rapid Eye Movement) sleep: sleep characterized by the random rapid movement of the eyes when derams also occur
    • Characterized by neocortical (in the neocortex) EEG waves similar to those when a person is awake
    • REM is accompanied by atonia
    • Atonia: paralysis of muscle
    • Dreaming occurs only in REM sleep
    • The first REM period lasts 10-15 min
  • Over the course of one’s lifetime: slow-wave sleep time decreases & REM time increases
  • Overall sleeping time at different life stages:
    • Infants: up to 18 hrs
    • Older adults: 6-7 hrs
    • Less time sleeping in general and in slow wave sleep specifically

Sleep Disorders

  • Insomnia: the most common sleep disorder where individuals have trouble falling asleep
    • Some people have problems falling asleep, some wake up in the middle of the night and can’t fall asleep again
    • Sleeping drugs do not help because they suppress slow-wave sleep and aren’t effective in keeping people asleep
  • Many common disorders disrupt deeper stages of sleep
  • Excessive daytime sleepiness has many causes
    • Obstructive Sleep Apnea: airway muscles relax and close airway causing difficulty breathing
    • individual wakes up without entering deeper stages of slow-wave sleep.
    • Causes high blood pressure and increases the risk of heart attack
    • more daytime sleepiness
    • Periodic Limb Movements: intermittent jerks of the legs or arms that occur as individual enters slow wave sleep and cause arousal from sleep.
    • REM behavior disorder: occurs when muscles fail to become paralyzed during REM sleep
    • Act out dreams by getting up and moving around.
    • Can be very disruptive.
    • Both periodic limb movements and REM behavior disorder are more common in people with Parkinson’s disease.
    • Can be treated with drugs for Parkinson’s or with a benzodiazepine called clonazepam
  • Narcolepsy: mechanisms controlling transitions into sleep (particularly REM sleep) don’t work.
    • Narcolepsy is caused by the loss of nerve cells in lateral hypothalamus that contain orexin/hypocretin.
    • Have sleep attacks during day (suddenly fall asleep).
    • Hypnagogic hallucination: individuals tend to enter REM sleep very quickly and enter dreaming state while partially awake
    • Cataplexy: loss of muscle tone similar to what happens in REM sleep but occurs when the individual is still awake

How is Sleep Regulated?

  • Wakefulness is maintained by systems in the upper brainstem and hypothalamus
    • Neurons here use acetylcholine, norepinephrine, serotonin, glutamate to connect with the forebrain
    • Neurons in the hypothalamus use orexin and some contain histamine
    • Thalamus and basal forebrain activation by acetylcholine is very important too
    • basal means “closest to midbrain/base”
  • Level of alertness can be shown in an activated low-volt EEG
  • Arousing systems are less active in non-REM sleep
    • Transmission of information to the thalamus is limited
  • Ventrolateral preoptic (VLPO) nucleus: area in the brain that causes suppression of arousal systems
    • VLPO nucleus neurons have the inhibitors galanin and GABA
    • Damage to the VLPO area produces irreversible insomnia
  • In REM sleep, there is an internally activated brain and EEG but the external input is suppressed
    • Internal activation comes from cyclically active REM sleep generator neurons in the brainstem
  • Signals from neurons cause the excitation of the forebrain
    • Leads to rapid eye movements & muscle suppression
  • Forebrain excitation driving force behind dreams of REM sleep
  • Motor cortex neurons fire as rapidly during REM sleep as during waking movement
    • Explains movement coinciding with dreams
  • Periodic recurrence of REM sleep
    • REM sleep occurs every 90 min during sleep
    • This is caused by on-and-off switching of REM-generators (acetylcholine, glutamate) and REM-suppressors (norepinephrine, serotonin, GABA)

The Sleep-Wakefulness Cycle

  • 2 determining factors for sleepiness:
    • Circadian system: Monitoring the time of day/night
    • Homeostatic system: monitoring how long the person is awake
  • Circadian system is regulated by the suprachiasmatic nucleus
    • Suprachiasmatic nucleus: a small group of cells in the hypothalamus serving as the master clock
    • It expresses clock proteins that go through a biochemical cycle of approximately 24 hrs
    • This sets the pace for daily cycles of activity, sleep hormone release, etc.
  • The suprachiasmatic nucleus also receives input from the retina
    • The clock can be reset by light so it is linked to the outside day-night cycle
  • Also provides information to the subparaventricular nucleus → dorsomedial nucleus → VLPO and orexin neurons
    • Orexin: an excitatory signal to arousal system especially norepinephrine neurons
    • Orexin activation plays role in preventing transitions into REM sleep during the day
  • Arousal mediated by orexin and activation of norepinephrine neurons in the locus coeruleus
    • Locus coeruleus: a lateral part of brain stem that has norepinephrine producing neurons that mediate arousal along with orexin
  • The homeostatic system responds to longer wake periods by increasing the urge to sleep
    • The longer a person is awake, the greater the likelihood of an increase in sleep-inducing factors
  • Adenosine: a very important sleep promotor
    • More adenosine means increased sleepiness
    • Adenosine release starts in the basal forebrain and spreads to the rest of the cortex
    • Increased adenosine levels slow down cellular activity and diminish arousal
    • Adenosine levels decrease during sleep
  • Brain adenosine may be produced by ATP breakdown over the course of wakefulness
  • Neuron activity decreases and adenosine levels decline in non-REM sleep
    • ATP levels increase during sleep